The invention relates to a low-pressure mercury discharge lamp having emission maxima in three spectral regions, provided with a gastight, radiation-transmitting discharge vessel with a gas filling comprising mercury and rare gas and provided with a luminescent layer which comprises at least a first luminescent material with an emission maximum in the 590-630 nm region and a second luminescent material with an emission maximum in the 520-565 nm region.
Low-pressure mercury discharge lamps with emission maxima in three spectral regions are known from U.S. Pat. No. 4,176,294 and Netherlands Patent 164,697. To achieve these maxima in three spectral regions, the low-pressure mercury discharge lamps comprise a first (red-)luminescing material with an emission maximum in the 590-630 nm region and a second (green-)luminescing material with an emission maximum in the 520-565 nm region. The required emission in the third spectral region, i.e. the 430-490 nm region, is supplied in many cases by a third (blue-)luminescing material. The visible radiation emitted by the mercury vapour discharge itself, however, also offers a contribution (i.e. the emission of the 436 nm mercury line) in this spectral region, so that the presence of a blue-luminescing material in the luminescent layer is not always necessary. The low-pressure mercury discharge lamps emit white light at a given colour temperature, i.e. the colour point (x, y in the CIE chromaticity diagram) of the emitted radiation is situated on or adjacent the Planckian locus. In addition to the requirements to be imposed on the luminescent layer as regards the properties mentioned above relating to the colour temperature of the light radiated by the low-pressure mercury discharge lamp the luminescent layer also has to meet particular requirements regarding colour rendering and luminous efficacy.
A red-luminescing material which is frequently used in practice is yttrium oxide activated by trivalent europium (YOX). This material complies satisfactorily with the requirements listed above. Low-pressure mercury discharge lamps comprising YOX in their luminescent layer are widely used for general lighting and have the advantage that they have a good general colour rendering (colour rendering index R(a,8) of at least 80). A disadvantage, however, is that this good general colour rendering is not necessary in every application of a low-pressure mercury discharge lamp while this good general colour rendering is generally achieved at the cost of the luminous efficacy of the lamp.